Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices may incorporate both a pacemaker and a defibrillator.
In general, the pacing function of implantable cardiac devices is provided by two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing and/or sensing electrode configurations. In the unipolar configuration, the pacing stimulation pulses are applied, or intrinsic responses are sensed, between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. The electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In the bipolar configuration, the pacing stimulation pulses are applied, or intrinsic responses are sensed, between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, with the most proximal electrode serving as the anode and the most distal electrode serving as the cathode.
Implantable cardiac devices deliver pacing pulses to the heart to induce a depolarization and a mechanical contraction of that chamber when the patient's own intrinsic rhythm fails. To this end, these devices include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring P waves and/or R waves, the cardiac device circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required, to help stabilize the electrical rhythm of the heart.
Pacing systems may function as single-chamber, dual-chamber, or biventricular systems. A single-chamber system stimulates and senses in one chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode. Biventricular systems stimulate in corresponding chambers of the heart as, for example, the right ventricle (RV) and left ventricle (LV).
Biventricular pacing has been shown to coordinate contractions of the left and right ventricles, reduce the amount of blood flow that leaks through the mitral valve, and decreases the motion of the septal wall that separates the chambers of the heart. Such motion can affect the quantity of blood that the ventricle can pump out in a single beat.
Biventricular pacing has been found to be particularly advantageous in patient's suffering from congestive heart disease because of the improved ability of the left ventricle to fully pump blood from the heart. As a result, patients are able to tolerate greater exertion, have a longer life span, and experience a higher quality of life.
Biatrial pacing has also been suggested to also lend in coordinating contractions of the right and left atria. As used herein, the term corresponding chambers is meant to refer to either the right and left atria or the right and left ventricle.
With the ability to pace either or both sets of corresponding heart chambers, it is believed that a wide variety of irregular heart conditions may be most efficiently addressed. For example, in a patient suffering from dilated cardiomyopathy, typically the left ventricle is predominately affected in the earlier stages of the disease. The dilated left ventricle has diminished contractility causing its contraction to be slower and weaker than the still healthy right ventricle. Thus, by selecting the stimulation pathway direction from the left ventricle to the right ventricle, the slower left ventricle contraction is initiated prior to the faster right ventricle contraction, yielding superior synchronization of right ventricle and left ventricle contractions.
Traditional unipolar pacing of the left ventricle entails applying a pacing pulse between a left ventricle tip electrode carried on a lead implanted in the coronary sinus of the heart and serving as the cathode and the conductive enclosure of the implantable cardiac stimulation device serving as the anode. The distance between these electrodes can require substantial energy to achieve reliable capture of the left ventricle. These energies could cause contraction of chest muscle resulting in discomfort to a patient.
To avoid such high pacing energies, cross chamber pacing has been performed. Here, to pace the left ventricle, for example, instead of using the device enclosure as the anodal pacing electrode, an electrode carried on a separate lead and implanted in the right ventricle, such as the right ventricular ring electrode, is enlisted as the left ventricular anodal pacing electrode. This has been found to lower pacing energy requirements for left ventricular pacing. However, it has also been found that anodal capture may occur at the right ventricular electrode thereby resulting in simultaneous pacing of the right ventricle with the left ventricle. If permitted, this would subvert the desire to pace the ventricles separately and, more particularly, the left ventricle before the right ventricle.
While anodal capture may be undesirable in the case of cross-chamber stimulation, it may prove beneficial in single chamber stimulation. For example, in the case of left ventricular pacing, simultaneous, multisite stimulation, i.e., both anodal capture and cathode capture, of the left ventricle may provide improved left ventricular contraction.
Accordingly, there is a need for cardiac stimulation devices and related methods that provide for anodal capture detection. In the case of cross-chamber stimulation, there is a further need for devices and methods that operate to prevent anodal capture. In the case of single chamber stimulation, there is a further need for devices and methods that operate to provide anodal capture. Various aspects of the present invention fulfill each of these needs.